How do potholes form, and why do they always happen in the spring?
Potholes are the holes in the roadway that can be various shapes and sizes caused by the expansion and contraction of water after it has entered into the subsurface under the pavement from a crack in the surface. When water freezes, it expands. Think of when ice cubes are made. A tray full of water is put into the freezer when you take the tray out of the freezer later, you will notice the water has expanded. This same effect happens when water gets into the subsurface under the pavement. If it has a chance to freeze, it will take up more space under the pavement, and then the pavement will expand, bend or crack, which weakens the material. Then when ice melts, the pavement contracts and it leaves gaps or voids in the subsurface under the pavement, where water can get in again. If the water freezes and thaws over and over, the pavement may get very weak.
There is another thing that happens. As the weight of cars and trucks pass over the weak spot in the road, pieces of the roadway material weakened by the freeze-thaw effect get displaced or broken down from the weight, creating the pothole.
What happens when salt is brought into the picture? Water will freeze at 32 degrees Fahrenheit. When salt is used, it lowers the temperature that water will freeze, creating an artificial freeze-thaw cycle to occur. This happens more in the spring because of the melting that takes place and because the temperatures go between above freezing and below freezing very frequently and allows many freeze-thaw cycles to weaken the pavement.
What causes corrosion on metal objects and how does salt or other deicing chemicals fit into the equation?
For corrosion to take place, there are three things that are needed before it happens — metal, air and water. Metal things are usually exposed to air, but when water gets introduced into the picture corrosion can begin. Water gets into an area of metal that is weak or not protected from paint or oil. An example of this is a barbed wire fence that you see along the highway. The metal is always exposed to air, but when the humidity in the air gets high or when it rains, slowly over time, the barbed wire fence will rust. So when there is a weak spot in the metal of your vehicle and water finds a way to get to the metal, corrosion will begin.
How does salt or deicing chemicals fit into this equation?
The salt or deicing chemical is a corrosion accelerator. It is not necessary for corrosion to take place, but it speeds the process of corrosion along. Therefore, when you see corrosion on a vehicle, it means the metal has been exposed to air and water. Salt from the road may have accelerated the corrosion process and made the corrosion worse.
What roads are the department responsible for plowing?
The department is responsible for all interstates and primary highways. There are two types of primary highways: U.S. highways, that are designated by a sign along the roadway with a number inside a shield. State highways, that are designated by a sign with a number inside a circle.
Why is it that I never seem to see a snowplow during a winter storm?
The department is responsible for snow removal for approximately 24,700 lane-miles of roadway. With 901 trucks, the average time to complete a snow route is approximately two to three hours, but some can take approximately as long as four hours. There is also the amount of time needed to load and unload the truck with deicing materials. The number of lane-miles if laid end-to-end would circle the earth.
How much salt is used in a typical winter season, and how much does it cost?
The department uses about 200,000 tons of salt each season, which means that we use about 0.20 pounds of salt per square foot of pavement during a winter season. The statewide average cost of salt for the 2009 winter season is about $67 per ton.
Why does the department have its own weather reporting stations?
The department has 62 specialized weather reporting stations collecting road surface information and atmospheric information that reflect conditions on the roadway. The systems measure air and pavement temperatures, relative humidity, wind speed and direction, subsurface temperatures, depth of precipitation on the roadway, and salt concentration.
What is the importance of pavement and subsurface temperatures? Why not use air temperatures?
Air temperatures are not usually good indicators of what the actual temperature is of the road surface. During the fall, the pavement is often kept warmer than the surrounding air because of the warm soil. During the spring, the reverse may be true and pavement temperatures can be colder than the air because the soil is still frozen from the long winter temperatures. The sun also has a strong influence on the pavement temperatures that will help heat the pavement and help the melting process. The difference between air and pavement temperatures can often differ by as much as 20 degrees Fahrenheit.
In the example above, on a bitterly cold early winter day when the air temperature was not even 4 degrees Fahrenheit, the pavement or surface temperature was 14 degrees Fahrenheit, primarily because the subsurface temperature had not yet cooled and remained at 32 degrees Fahrenheit.
During some winter storms it seems like the department does a good job, but during other storms it seems that they are not doing a very good job. Why the difference in performance?
One of the biggest factors that determine performance is the type of storm and temperatures. Storms with low temperatures can be difficult because deicing chemicals become less effective at the lower temperatures. It takes nearly eight times as much salt to melt a pound of ice at 20 degrees Fahrenheit then at 30 degrees Fahrenheit. Storms with high winds are a challenge because the snow quickly blows back onto the roadway after the plows pass. There are reportedly more than 65,000 combinations of winter storms that can hit Iowa during the winter, and each pose unique problems to snowplow operators.
Why are you spraying water on the roadway on a perfectly clear day?
Spraying a liquid salt-brine solution on the roadway will help keep snow and ice from bonding to the pavement. Spraying a salt solution on the roadway is similar to spraying a frying pan with oil to keep food from sticking to the bottom of the pan. The salt solution acts as a barrier so the snow and ice does not form a strong bond to the pavement. Studies show that under extremely cold conditions ice frozen to concrete has a stronger bond than concrete alone. In many locations the Iowa DOT also sprays the salt solution on bridge decks the afternoon before a predicted frost. The early application of the salt solution helps prevent frost from forming on bridge decks throughout the night.
How many snowplows does the department have?
The department has 901 snowplows available throughout the state. These plows are real workhorses when you look at how much snow they actually push during a normal winter. Iowa averages approximately 32-34 inches of snow each year, and if you consider that an average cubic foot of snow weighs about 8 pounds (varies between 6-12 pounds depending on moisture content), this means each truck in the fleet pushes 18,700 tons of snow each year and statewide they push 18.5 million tons. For comparisons, the Panama Canal was constructed over a 33-year period removing 400 million tons of rock and earth while the same amount of snow is removed by the department in a little more than 20 years.
What hours do the plows maintain during a storm?
Typically plows are operating 24 hours a day, but at times reduced numbers may be available as operators are given time off for rest. If weather conditions are so severe that progress is not being made, trucks may be pulled off the road until conditions improve.
Who is responsible for the Winter Road Condition Report that I see on the news and Internet?
The Iowa State Patrol, a division of the Iowa Department of Public Safety, is responsible for the Winter Road Condition report. See current road conditions on www.511ia.org.
What is the typical size of trucks in the department's fleet?
The department has two basic categories of trucks used in winter operations; heavy duty and medium duty. Heavy duty trucks have a capacity of 8 cubic yards, and the medium duty's capacity rating is 4 cubic yards. Trucks are usually kept for about 15 years and then sold at auction.
How fast does a snowplow travel?
Typically when the plow is down snowplows operate at 20-35 mph, but are often moving even slower if the snow is deep or conditions warrant a slower speed. Always be on the lookout for the yellow flashing warning lights during a winter storm because the lights warn you of a slow-moving snowplow.
Who is responsible for plowing snow on a highway in the city?
It could be the Iowa DOT or the city. In several communities agreements have been made between the Iowa DOT and the city to have the city remove snow and ice on the highway. These agreements can help reduce costs to the Iowa DOT and provide for more continuity of service on the highways.